1. Field of the Invention
The present invention relates to a motor-driven power steering system for vehicles such as automobiles, and more particularly to a motor-driven power steering system having a steering servo unit including an electric motor for producing assistive steering torque.
2. Description of the Relevant Art
Various electric or motor-driven power steering systems for automobiles have been proposed in recent years in view of the structural complexities of conventional hydraulically operated power steering systems.
One example of such an automotive motor-driven power steering system is disclosed in UK patent application No. 2,132,950 A published on July 18, 1984. The disclosed motor-driven power steering system has a steering servo unit using a low-torque, high-speed electric motor as a power source and a control apparatus for the steering servo unit. When a steering wheel is turned, the steering torque applied to the input shaft of the steering system which is coupled to the steering wheel is detected, and the motor is controlled by the detected steering torque. Assistive torque produced by the motor is transmitted via a speed reducer to the output shaft of the steering system. The speed reduction ratio of the speed reducer is selected to be high since the motor rotates at high speed. The assistive torque applied to the output shaft of the steering system helps the driver turn the steering wheel with reduced manual forces, resulting in improved drivability and steering feeling.
General automotive steering systems including manually operated steering systems have two modes or states. In one state, the driver imposes steering forces on the steering wheel, and in the other state, the driver applies no steering forces to the steering wheel. While the automobile is turning with steerable or dirigible wheels, which are front wheels in most cases, being steered in one direction, the front wheels are subjected to a force tending to return themselves back to their central or neutral position. Such a returning force is produced by the front wheel alignment or self-aligning torque arising from elastic deformation of the front wheels. The returning force is increased as the speed of the automobile becomes higher. When the driver stops applying the steering force to the steering wheel, with his or her hands released or not, at the time the dirigible wheels have been steered a certain angle, the steered wheels are apt to return to the neutral position. At the same time, the steering wheel also tends to return to its neutral position. Such a returning state will hereinafter be referred to as a "freely returning state".
In manually operated steering systems with no steering servo unit, the steering angle varies according to a curve L1 of FIG. 8A of the accompanying drawings during the freely returning state of the steering wheel. In FIG. 8A, the vertical and horizontal axes represent the steering angle .theta. and time t, respectively. The graph of FIG. 8A is plotted under such conditions in which the driver stops applying the steering force to the steering wheel when the steering wheel has been turned .theta.i clockwise from the neutral position (.theta.=0) at a certain automobile speed. As shown in FIG. 8, the steering wheel repeatedly overshoots the neutral position until finally it returns or settles to the neutral position in a time tm.
Now, it is assumed that the steering wheel of the motor-driven power steering system, as described above, enters the freely returning state under the same conditions as those described above with respect to the manually operated steering system. At this time, the motor is rotated by the steered wheels through the speed reducer, and hence acts as a load on the steered wheels. As a result, the rate of change of the steering angle per unit time is smaller than that of the manually operated steering system. Stated otherwise, the period of reciprocating rotational movement of the steering wheel is longer than that of the manually operated steering system. Furthermore, since the moment of inertia of the motor acts on the steered wheel at a rate which is the square of the speed reduction ratio of the speed reducer, the overshooting of the steering wheel from the neutral position is larger than that of the manually operated steering system. Under the freely returning state of the steering wheel, the steering angle .theta. changes according to a curve L2 of FIG. 8B. The settling time Te in which the steering wheel returns to its neutral position is considerably longer than the settling time Tm of the manually operated steering system. Thus, the steering wheel of the motor-driven power steering system returns to the neutral position relatively slowly.
The present invention has been made in an effort to solve the aforesaid problems of the conventional motor-driven power steering system.